The present invention relates to a fluid pressure control system for controlling an automatic transmission which is used in an automotive vehicle, and, more particularly, relates to an improvement in such a fluid pressure control system for more suitably conforming the characteristics of the shifting performance of the transmission to the torque output characteristics of the engine of the vehicle.
Automatic transmissions for automotive vehicles are well known. They typically include a gear transmission mechanism and several friction engaging mechanisms such as clutches and brakes. By the selective engagement and disengagement of the friction engaging mechanisms, which are typically actuated by fluid pressure, various speed stages are made available from the gear transmission mechanism.
Various fluid pressure control systems are known for providing actuating fluid pressures for the friction engaging mechanisms. Typically, such a fluid pressure control system includes a throttle fluid pressure control valve, which produces a throttle fluid pressure which varies linearly according to a quantity representing the load on the engine of the vehicle, which is usually, in the case of an engine fitted with a carburetor, the opening amount of the throttle valve of the carburetor; a governor fluid pressure control valve which produces a governor pressure which varies linearly according to the road speed of the vehicle; and at least one shift valve, which shifts according to a balance between the throttle pressure and the governor pressure, and thereby selectively supplies fluid pressure to, and exhausts fluid pressure from, at least one of the aforesaid plurality of friction engaging mechanisms.
Such a friction engaging mechanism generally comprises two groups of friction plate elements, these two groups being mutually interleaved, and being respectively connected to elements of the gear transmission mechanism and/or the housing thereof which are mutually rotatable. Further, a fluid pressure activated servo device is typically provided, supply of operating fluid pressure to which squeezes together the two groups of friction plate elements so that they engage together and transmit torque between the aforesaid mutually rotatable parts of the transmission. In more detail, when the servo device is not compressing together the two groups of friction units, they are able substantially freely to rotate with respect to one another; and, as operating fluid pressure supplied to the servo device progressively increases, the groups of friction plate elements are progressively more and more strongly squeezed together, and accordingly their maximum torque transmitting ability progressively increases, in approximately direct proportion to the magnitude of the operating fluid pressure provided to the servo device. Finally, when the operating fluid pressure provided to the servo device reaches a sufficiently high level, the two groups of friction plate elements are locked together, and are securely engaged together, so that they cannot rotate with respect to one another.
A major problem with regard to prior art fluid pressure control systems has been the ensuring of smooth engagement of such a friction engaging mechanism, in order for the transmission to change speeds without producing any transmission shift shock, over a wide range of operating conditions of the automatic transmission. In more detail, a particular speed shift may occur at various different vehicle road speeds and at various different engine load conditions, i.e., various different throttle openings. Generally speaking, the greater is the load upon the engine of the vehicle during shifting of transmission speed stage, the greater will be the torque which is required to be handled by the friction engaging mechanism which is being engaged. Accordingly, it has been practiced for the transmission line fluid pressure, which is typically the source of fluid pressure which is selectively switched by the aforementioned shift valve and which is output as an actuating fluid pressure for the servo device of the friction engaging mechanism, to be linearly increased according to increase of the load on the engine of the vehicle. In more detail, it has been practiced for the line fluid pressure to be at a predetermined base level when the load on the engine is zero or very low, and to be increased from this base level by an amount substantially proportional to the load on the engine. According to this, the actuating fluid pressure for the servo device of the friction engaging mechanism, which is this line pressure as controlled by the shift valve, is thereby also increased according to increase of the load upon the engine, and thereby is higher when the servo device of the friction engaging mechanism is required to control a higher torque.
In order smoothly to mediate the shifting of gear shift stage of the transmission, and in order to make the engagement and the release of a friction engaging mechanism more gradual and more smooth, it is common in the prior art to connect a back pressure type accumulator to an intermediate portion of the fluid passage which conducts the actuating fluid pressure from the abovementioned shift valve to the servo device of the friction engaging mechanism. Generally speaking, such an accumulator comprises a cylinder and a piston reciprocating within the cylinder. A chamber defined on the one side of the piston serves as a fluid accumulator chamber, and is communicated to said intermediate portion of said passage, so as temporarily to accumulate fluid therefrom in order to condition sudden surges of fluid pressure therein; and a chamber defined on the other side of the piston serves as a back pressure chamber, and is connected to a source of fluid pressure such as line fluid pressure or throttle fluid pressure, which thus elastically supports the piston against the pressure in said passage which is communicated to the accumulator chamber.
For the friction engaging mechanism to be smoothly engaged and disengaged under the mediating effect of such a back pressure type accumulator as connected to an intermediate portion of the fluid passage supplying operating pressure to the servo device of the friction engaging mechanism, the fluid pressure supplied to the back pressure chamber of the accumulator should be suited to the magnitude of the torque required to be controlled by the friction engaging mechanism. That is to say, if the engine of the vehicle is producing a high torque, which is generally the case when the load on said engine is high, it is desirable that the back pressure supplied to the back pressure chamber of the accumulator should be higher than when the engine is only producing a medium or low torque. In the prior art, the back pressure supplied to the back pressure chamber of such an accumulator has typically been line fluid pressure or throttle fluid pressure, both of which, as explained above, have increased linearly with the load upon the engine of the vehicle. However, the torque delivered by the engine of the vehicle does not increase strictly linearly with the load. In fact, the graph of torque against load for a typical internal combustion engine is convex upwards, i.e., the rate of increase of torque with increase of load decreases according to increasing load. This difference in variation characteristic with respect to load on the engine, between the line pressure or the throttle pressure and the torque delivered by the engine, is quite significant, and has made it impossible in the prior art effectively to conform the fluid pressure supplied to the back pressure chamber of such an accumulator to the torque which is required to be controlled by the friction engaging mechanism whose engagement is mediated by such an accumulator. In other words, if the back pressure provided to the back pressure chamber of such an accumulator is arranged to be properly in conformity with the torque being delivered by the engine in the region of a particular engine load which is required to be controlled, then in other ranges of engine load this conformity between back pressure and controlled torque will no longer be the case. In fact, in the prior art, it has been impractical to conform the back pressure supplied to the back pressure chamber of a fluid accumulator for mediating the engagement of a friction engaging mechanism to the torque required to be controlled by the friction engaging mechanism, over a wide range of engine load.